1. Field of the Invention
The present invention relates generally to wireless networks, and more specifically, to a method and system for synchronizing a location finding system within a wireless network.
2. Background of the Invention
A multitude of wireless communications systems are in common use today. Mobile telephones, pagers and wireless-connected computing devices such as personal digital assistants (PDAs) and laptop computers provide portable communications at virtually any locality. Wireless local area networks (WLANs) and wireless personal area networks (WPANs) according to the Institute of Electrical and Electronic Engineers (IEEE) specifications 802.11 (WLAN) (including 802.11a, 802.11b, etc.), 802.15.1 (WPAN) and 802.15.4 (WPAN-LR) also provide wireless interconnection of computing devices and personal communications devices, as well as other devices such as home automation devices.
Within the above-listed networks and wireless networks in general, it is desirable to know the location of devices for operation of location-based services, mapping of network facilities, and security. The above-incorporated parent application describes a method and system for location finding within a wireless network and various applications for the location finding results. The method and system use the time-difference-of-arrival (TDOA) of a transmitted signal received at multiple receivers within a wireless network in order to determine the position of the transmitting device via triangulation.
In the TDOA location finding system described above, synchronization of devices is essential so that accurate TDOA measurements are made between the network devices. Synchronization in the above-referenced parent application is achieved through synchronizing a timebase in each of the location-finding units.
Hard-wired synchronization schemes may distribute a clock from a central source, but distribution of a clock is a costly alternative to synchronization. Hard-wired schemes that pass synchronization error information over a wired network connection as well as wireless synchronization schemes are preferable, but rely on stability and accuracy of the local clock in each location receiver.
Synchronization of local-clock timebases via a synchronization message consumes valuable network bandwidth and the accuracy of the message-based synchronization scheme described above is dependent on the frequency of synchronization messages. There is an upper limit on the accuracy of the above-described synchronization scheme determined by how frequently synchronization messages can be transmitted and received. Therefore, it would be desirable to provide a method and system for accurately measuring location within a TDOA-based location finding system that introduces little or no synchronization overhead or distribution of a wired high-frequency clock or accurate edge reference signal to the location receivers.